1. Field of the Invention
The present invention relates to a fuel cell system and an operation method thereof. More particularly, the present invention relates to a polymer electrolyte fuel cell system and an operation method thereof.
2. Description of the Related Art
In recent years, concern about environmental problems has been increasing on a global scale, under the influence of global warming due to an increase in carbon dioxide concentration or acid rain or the like due to an increase in emission of exhaust gases. So, in a field of power supply development, attention has been focused on a fuel cell system capable of energy change which is highly efficient and keeps the environment clean without emission of carbon dioxide. Among various fuel cell systems, particular attention has been paid to a polymer electrolyte fuel cell system that operates at a low temperature and has high output density, which is expected to be used as civil power supply, power supply for power-driven automobile, etc.
The polymer electrolyte fuel cell system is one type of fuel cell system that employs a membrane electrode assembly (hereinafter simply referred to as MEA) in which chemical reaction for power generation takes place. The polymer electrolyte fuel cell system typically includes a polymer electrolyte fuel cell stack (hereinafter simply referred to as stack) structured such that individual polymer electrolyte fuel cells (hereinafter simply referred to as cells) are stacked in predetermined number, and predetermined auxiliary equipment that operates the stack, which will be described later. Hereinbelow, construction of the cells, the stack, and the polymer electrolyte fuel cell system will be sequentially described.
The cells forming the stack are each provided with the MEA in which catalytic reaction for power generation takes place. The MEA includes a pair of catalyst layers (anode catalyst layer and cathode catalyst layer) provided on surfaces of both sides of a polymer electrolyte membrane that selectively transports hydrogen ions and formed to contain carbon powder carrying platinum-group metal catalyst thereon, and a pair of gas diffusion electrodes (anode gas diffusion electrode and cathode gas diffusion electrode) provided to sandwich the pair of catalyst layers between them and chiefly made of carbon fibers. The gas diffusion electrodes have both gas permeability and electron conductivity. Further, sealing gaskets are disposed to sandwich a peripheral portion of the polymer electrolyte membrane of the MEA, thereby forming a MEA-gasket assembly. The MEA-gasket assembly is sandwiched between an anode separator provided with a fuel gas passage through which a fuel gas (hydrogen or hydrogen-rich gas) flows and a cathode separator provided with an oxidizing gas passage through which an oxidizing gas (air) flows, thereby forming the cell.
As stated above, the stack is comprised of cells stacked in predetermined number. The reason why the stack is formed in the polymer electrolyte fuel cell system is that the electromotive force of the cell is low as approximately 0.6 to 0.8V in a rated operation range, although this depends on an output current density. So, by stacking cells to form the stack, a voltage sufficient to operate electronic equipment or the like is gained. Typically, this stack is formed by stacking cells of several tens to several hundreds. This stack generates heat according to the number of stacked cells, because the cells are generating heat during power generation. Since the density of heat generation of the stack is higher relative to a single cell, a cooling water passage is typically provided in every one to three cells to allow the stack to be forcibly cooled by using a cooling medium such as water or ethylene glycol. The use of the cooling medium allows the temperature of the stack generating heat to be kept in a suitable condition. So, three types of fluids, i.e., fuel gas, oxidizing gas, and water (or, e.g., ethylene glycol) or the like are supplied to the stack. A pair of (or plural pairs of) manifolds (common holes) are provided on the cathode separator and the anode separator for each of these three fluids. Each fluid is introduced from the manifold into grooves provided on the separator and branches to flow within the cell and the water cooling portion. For example, the fuel gas is introduced from a fuel gas supply manifold into a fuel gas passage of the anode separator. While flowing within the fuel gas passage, the fuel gas is consumed in catalytic reaction for power generation in the MEA. The excess fuel gas remaining unconsumed after power generation is exhausted through a fuel gas exhaust manifold. And, the cells and the water cooling portions are alternately stacked into a stack, which is then sandwiched between end plates with current collecting plates and insulating plates interposed between the stack and the end plates. Thereafter, these are fastened from both ends by fastening bolts, thereby manufacturing a typical stack.
The polymer electrolyte fuel cell system refers to a whole power generation system configured to operate the stack to thereby take out a predetermined electric power. Specifically, the polymer electrolyte fuel cell system comprises, as components configured to directly drive the stack, a reformer configured to convert available fuel precursor such as LPG, LNG, or gasoline into a fuel gas through a steam reforming reaction, a fuel gas supply device configured to supply the reformed fuel gas to the stack, an oxidizing gas humidifier configured to humidify air used as an oxidizing gas, an oxidizing gas supply device configured to supply the humidified oxidizing gas to the stack, a cooling water supply device configured to supply circulation cooling water into the stack, an electricity loading device configured to load an electric power, and the like. Typically, the fuel gas is humidified in such a manner that water is added to the fuel gas by a steam reforming process. Also, typically, the oxidizing gas is humidified in such a manner that total enthalpy heat exchange is conducted between the oxidizing gas (hereinafter referred to as a cathode exhaust gas) exhausted from the stack and the oxidizing gas supplied from an air supply device by utilizing water contained in the oxidizing gas exhausted from the stack to allow the oxidizing gas from the air supply device to be humidified to a desired state. This total enthalpy heat exchange is conducted using a total enthalpy heat exchange membrane that permits passage of water but does not permit passage of gases. As the total enthalpy heat exchange membrane, a polymer electrolyte membrane used in the cell (e.g., perfluorosulfonic acid) is suitably employed. And, the above components and the stack are connected to one another to be constructed into the polymer electrolyte fuel cell system.
Herein, the outline of a power generation principle of the cell in the polymer electrolyte fuel cell system will be described.
In the above constructed cell, the fuel gas is supplied to the fuel gas passage of the anode separator, while the oxidizing gas is supplied to the oxidizing gas passage of the cathode separator. Thereby, the fuel gas is exposed to a principal surface of the MEA on the anode catalyst layer side and the oxidizing gas is exposed to a principal surface of the MEA on the cathode catalyst layer side. At this time, the fuel gas flows through the fuel gas passage of the anode separator and further through the anode gas diffusion electrode and contacts the anode catalyst layer provided on the MEA. Through a catalytic reaction in the anode catalyst layer, the fuel gas is dissociated into hydrogen ions and electrons. The dissociated electrons travel through the anode gas diffusion electrode and are collected into the anode separator. Then, the electrons are supplied to electronic equipment or the like connected to the polymer electrolyte fuel cell system. Meanwhile, the dissociated hydrogen ions travel to the cathode catalyst layer through an inside of the polymer electrolyte membrane. In the cathode catalyst layer, the hydrogen ions are consumed in a catalytic reaction for generating water, along with the oxidizing gas that passed through the cathode gas diffusion electrode and reached the cathode catalyst layer and the electrons that traveled to the cathode separator via the electronic equipment connected to the polymer electrolyte fuel cell system, traveled through the cathode gas diffusion electrode and reached the cathode catalyst layer. Through the above series of catalytic reactions, the electrons are continuously derived from the fuel gas. Thereby, the cell functions as a battery.
The polymer electrolyte membrane exhibits a stable hydrogen ion transport ability under a sufficiently moist condition. To this end, in operation of the polymer electrolyte fuel cell system, it is necessary to supply water to moisten the polymer electrolyte membrane. Typically, this water is supplied by supplying the fuel gas and the oxidizing gas in humidified state to the cell. In order to allow the catalytic reactions to progress well, it is necessary to heat the stack up to a temperature of at least 60° C. or higher, more preferably, at 60 to 80° C. To this end, in the polymer electrolyte fuel cell, power generation operation is carried out while heating the stack at a temperature of 60 to 80° C.
In order to operate the cell properly, it is necessary to keep the polymer electrolyte membrane in a sufficiently moist state as stated above. In addition, it is necessary to inhibit “flooding” (phenomenon in which the anode catalyst layer and the cathode catalyst layer are closed by the water) from occurring on the anode catalyst layer and the cathode catalyst layer due to the water generated in the MEA. This is because, if the flooding occurs, the catalytic reaction in which the fuel gas is dissociated into hydrogen ions and electrons and movement of the dissociated hydrogen ions to the cathode side within the polymer electrolyte membrane are difficult to progress, thereby significantly reducing the amount of electric power generated in the cell.
Accordingly, there has been proposed a method in which, in order to inhibit occurrence of the flooding, a pressure loss (pressure drop) is applied between supply (inlet) portion and exhaust (outlet) portion of the fuel gas and between supply portion and exhaust portion of the oxidizing gas so that dew points of the fuel gas and the oxidizing gas are always kept at not higher than an operating temperature of the stack, thereby exhausting excess water outside the cell (e.g., Japanese Laid-Open Patent Application Publication No. 04-502749). This will be described specifically. Since water absorbing ability of gases increases with decreasing pressure, it is possible to effectively exhaust water generated within the cell through the catalytic reaction outside the cell by setting pressures of gases existing at positions closer to the exhaust portions of the fuel gas and the oxidizing gas lower. That is, since the water generated within the cell may evaporate into the fuel gas and the oxidizing gas by keeping the fuel gas and the oxidizing gas at dew points not higher than the stack operating temperature, it is possible to exhaust the excess water from the cell along with excess fuel gas and oxidizing gas. In this case, since the water easily permeates the polymer electrolyte membrane, it is possible to exhaust excess water outside the cell along with excess fuel gas by keeping the fuel gas at a dew point not higher than the stack operating temperature so that the water generated on the cathode side is diffused through the polymer electrolyte membrane and then evaporates into the fuel gas. The pressure drop of the fuel gas or the oxidizing gas may be realized by providing an orifice in the supply portion of the gas, by extending the gas passage, by changing passage cross-sectional area, by increasing a friction coefficient of at least part of an inner surface of the gas passage, or setting a flow rate of the fuel gas within the passage higher than the amount of the fuel gas dissociated into hydrogen ions and electrons on the anode side.
In accordance with the above described conventional method in which the fuel gas and the oxidizing gas are always kept at the dew points not higher than the stack operating temperature, it is possible to effectively exhaust the water generated on the cathode side outside the cell, and to inhibit the flooding in which the anode catalyst layer and the cathode catalyst layer are closed by the water. In other words, the catalytic reactions can progress smoothly in both the anode catalyst layer and the cathode catalyst layer, and a proper amount of electric power can be generated in the cells.